Effective and practical methods for providing the required short circuits for waveguide and coaxial transmission systems have existed for many years; however, for microwave printed circuit or stripline systems using dielectric substrate support for the printed circuit, totally satisfactory methods are not as yet in general use.
Basically the stripline circuit in its simplest configuration consists of two layers of dielectric substrate material having copper ground planes on their nonadjacent sides, the dielectric substrate materials being passed tightly together with a printed circuit or stripline transmission circuit interposed between. To short such a transmission line at microwave frequencies, it is necessary to completely ground the transmission line to each of the spaced ground planes fastened to the dielectric substrates.
A number of techniques have been or are being used to ground the transmission line to the ground planes or to a metallic housing. One common techique is to solder a tab onto a printed circuit or stripline conductor which is placed between two dielectric substrates having ground planes on their opposite sides. This tab, which extends beyond the substrate members, is clamped under a shorting bar and screwed down or otherwise fastened tightly to the conductive housing. The shorting bar is then held down by the housing cover, it being of such height that the cover exerts a substantial force against it as it is secured. The difficulty with this arrangement is that air gaps tend to be formed adjacent the tab which is soldered to the stripline or printed circuit and also between the ground plane and the cover adjacent the shorting bar. Another technique which has been used is to solder a somewhat elongted tab to the transmission line, wrapping it around and soldering it down to one of the ground planes on the opposite sides of the dielectric substrate. An additional copper layer is then soldered to this wrap and also to the opposite dielectric substrate. Again, this arrangement ususally results in an undesirable air gaps between the substrates caused by the thickness of the tab soldered to the conductor. A further problem with this arrangement is that there is no means associated with this arrangement for positively controlling the position of the substrate and conductor within the support chassis. An additional problem is that since this results in a bonded or sealed assembly, the maintenance of the associated circuitry becomes more difficult. A further arrangement involves riveting the substrates together with rivet heads soldered to each of the ground planes on opposite sides thereof and with an additional inside washer soldered to the transmission line and to the rivet. With this arrangement, the electrical location of the short is not accurately predictable. There is also a high leakage of microwave fields, and the performance is generally very poor at frequencies above 10 GHz.
In addition to the above, there are also substantial problems in producibility or repeatability in production rising out of the above techniques. These problems become quite severe when operating at frequencies above 10 GHz. For example, it will be appreciated by those skilled in the art that in the manufacture of microwave filters using tuning stubs at this frequency range, it is necessary that the shorts or grounds be precisely and predictably located since very small variations in effective tuning stub length will result in altering of the frequency characteristics of the filter.